The present invention generally relates to vehicle stability control, and more particularly relates to a method and a device for controlling the traction slip of a vehicle on a roadway with sidewise different coefficients of friction.
Traction slip principally means that one or more vehicle wheels rotate faster than a wheel speed that corresponds to the current driving speed of the vehicle. This is mostly the case when the driving torque exceeds the torque which can be transmitted between the wheels and the roadway due to the frictional conditions prevailing between them.
FIG. 1 is a schematic view of a vehicle from the top. Reference numeral 11 relates to the left front wheel, 12 to the right front wheel, 13 to the right rear wheel, and 14 to the left rear wheel. When the vehicle has a front-wheel drive, traction slip can occur at wheels 11, 12. To reduce traction slip, it is either possible to reduce the engine torque, or brake pressure can be built up to decelerate the spinning wheels. The latter action is chosen especially in the presence of low rotational speeds when the reduction of the engine torque is scarcely possible.
Comparatively unfavorable driving situations may exist. For example, the sides of a roadway may be split with respect to coefficients of friction, e.g. in such a way that the right-hand wheels run on a low coefficient of friction (roadway is wet, full of sand, slippery), while the left-hand wheels run on a high coefficient of friction (roadway is dry). In FIG. 1, this is represented by hatched lines 17 which is meant to show the roadway areas having a low coefficient of friction. Without traction slip control, the case may occur that the driven wheel on a low coefficient of friction (e.g. 12) spins, while the wheel on a high coefficient of friction is hardly driven due to the effect of the differential. Traction is poor then and destabilizing.
Wheel-related traction slip control systems are known in the art (for example BTCS=xe2x80x98brake traction control systemxe2x80x99) which control the wheel slip on each individual wheel. Sometimes high traction slip values are required in difficult situations during starting-to-drive maneuvers which must not be suppressed in terms of control in order not to render starting to drive impossible. If in such unfavorable situations, for example, when driving uphill on split coefficients of friction, traction slip develops on the high coefficient-of-friction side as well, the build-up of brake pressure will impair the comfort and, possibly, also cause loss in traction.
On the other hand, there are traction slip control systems which control the traction slip on one wheel only, but in doing so also consider the behavior of another wheel (for example, EDL=xe2x80x98electronic differential lockxe2x80x99) under certain conditions. The objective of these systems in terms of control, especially in the presence of major differences in coefficients of friction, is to develop brake pressure only on the side having a low coefficient of friction, in order to make the entire engine output available for traction. Admittedly, these systems control the brake pressure on the low coefficient-of-friction side also in dependence on the behavior of the driven wheel on the high coefficient-of-friction side. However, when traction slip develops on the high coefficient-of-friction side, the shortcoming involved is loss in traction due to a considerable reduction of the supporting torque and a reduced comfort due to a high amount of traction slip on the low coefficient-of-friction side. No intervention is made on roadways having the same coefficients of friction on both sides and, therefore, approximately the same amount of traction slip on all driven wheels.
An object of the present invention is to provide a method and a device for controlling the traction slip of a vehicle on a roadway with sidewise different coefficients of friction which permit maximizing traction and involve least possible impairment of comfort.
Initially, the driving situation on a roadway with sidewise different coefficients of friction is identified. Identification may be performed, for example, by comparing the amounts of slip on the wheels of the driven axle. When the amounts of slip are obviously different, this indicates a driving situation with split coefficients of friction. Instead of the amounts of slip, however, the respective wheel speeds may also be chosen for consideration (because the amounts of slip are respectively calculated as a difference between the wheel speed and the vehicle reference speed).
To counteract loss in traction, the brake pressure on the driven wheel on the low coefficient-of-friction side may be decreased when traction slip occurs on both wheels of the driven axle. Although this causes higher traction slip on the low coefficient-of-friction side, it also produces (due to the differential) a lower slip amount on the high coefficient-of-friction side, in the most favorable case, it effects new grip of this wheel and, hence, an improved traction, a constant drive and, thus, driving comfort which is reduced only inconsiderably.
A traction slip control device includes an identification device for identifying the split coefficients of friction on the roadway and a brake actuation control which reduces the brake pressure on the low coefficient-of-friction side in the event of split coefficients of friction and traction slip that prevails on both sides. The brake actuation control may be a part of a comprehensive brake control system which fulfills different control objectives such as that of an anti-lock system or electronic stability optimization. In particular, the traction slip control according to the present invention may be a part of a prior art traction slip control system. It is preferably configured as a supplement to traction slip control on each individual wheel. Thus, it may e.g. add to the BTCS mentioned hereinabove.